Prior to feeding an electric power to a load device, the load device is electrically separated, a passive element is connected in parallel, and the output voltage of a variable voltage high-frequency power source is set to a lower voltage Vm for measurement, and then an output current value Im of the variable voltage high-frequency power source is measured. Using these Vm and Im, a voltage VIN of the variable voltage high-frequency power source is calculated for setting to a target value the output voltage of a secondary coil or of a rectifier when the load device is connected to the secondary coil. A non-contact power feeding apparatus is known, which controls, without exchanging information between the primary side and the secondary side by a communication device, the voltage of a variable voltage high-frequency power source 1 so as to set the voltage of the secondary coil or the output voltage of the rectifier to a target value (Patent Literature 1).